Most forming fabrics within the paper, cellulose and asbestos-cement industries must be woven endless. As a rule, the fabrics are made in endless form during the very weaving while using the so-called tubular weaving technique, or else the fabric ends are interconnected during the manufacture, e.g. by splicing. Usually, the splicing can be effected only by the manufacturer during the manufacture of the fabric. On account of the machine construction, some forming fabrics may be end joined only in connection with their mounting in the machine, and in such cases the fabrics must be provided with some suitable joint or seam fastener. Fabric end joining in immediate connection with the installation in the machine is most commonly used in the case of dryer felts and dryer cloths. In these types of products so called belt fasteners are used. The loops of these fasteners are of metal and usually U-shaped. They are attached either on the cloth itself or on separate fastener strips which are sewn onto the cloth ends.
In addition to the dryer section there are, however, several other positions in which it is necessary to deliver an open cloth the ends of which are to be interconnected directly in the machine. For instance, in Inverform machines the cloth does not as a rule lend itself to endless mounting. As belt fasteners are unsuitable for use to join together forming fabrics and certain filter types, a number of alternative possibilities have been developed during recent years to replace the belt fasteners. The one that has found most practical use is the so called loop seam wherein the warp thread ends are removed and thereafter are re-introduced into the weave, after having been formed into locking loops at the cloth end. The locking loops at both ends are then interconnected during the cloth mounting by means of one or several fastening wires. A seam of this kind, performed in a single-layer structure, is described for instance in the Swedish Published Specification No. 322 980.
In forming fabrics, it is important that the seam area has approximately the same openness as the rest of the fabric in order to avoid too pronounced marking and to reduce the risk of web breaks. It is likewise important that the locking loops are designed so as to avoid that they themselves or the warp threads in connection therewith are exposed to more wear than the rest of the fabric.
In later years, double-layer forming fabrics comprising two layers of weft threads and warp threads interconnecting these layers have found increasingly wide applications. Examples of fabrics of this kind are described in the two Swedish Published Specifications Nos. 366 353 and 385 486. Attempts have been made earlier to provide also the type of fabrics defined in these publications with loop seams, thereby making them useful in positions where the fabrics must be joined directly in the machine. On account primarily of their non-symmetric pattern, double-layer fabrics lack the natural properties that would make them suitable for provision of loop seams thereon. The reason therefor is that when the warp threads have formed a loop and are to be re-introduced into the cloth, the re-introduction must be performed in a manner ensuring that the crimp of the thread conforms with the pattern of the cloth. In the past, many loops therefore were either too small, thus making it impossible to pass a locking wire through them, or else too large, whereby openings formed between the locking wire and the rest of the fabric, causing increased marking and also risks of web breaks. Another disadvantage is that the warp thread in connection with the loop formation is exposed on the wear side of the fabric, resulting in increased wear.